1. Field of Invention
This invention relates to small waterplane area twin hull (swath) vessels also known as semisubmerged ships, specifically to an elevating platform apparatus attaching at the stern of a swath vessel which can be raised and lowered from a position nearly level to a vessel's deck to a position nearly level to the waterline allowing convenient movement of people and equipment from the deck of the vessel to the water.
2. Description of Prior Art
Small water plane area twin hull (swath) ships also called semisubmerged ships have been developed for improved motion characteristics in high sea states often at high speeds. Many U.S. patents including U.S. Pat. No. 234,794 to Lundborg (1880), U.S. Pat. No. 3,063,397 to Boericke (1962), U.S. Pat. No. 3,623444 to Lang (1971) and U.S. Pat. No. 4,174,671 to Seidl (1979) have been granted which disclose ships of this configuration.
Each of the configurations includes submerged pontoons that are parallel to each other and the design water line, vertical struts of reduced water plane area and an upper bridging deck. Optimal performance for these vessels is when their pitch, heave and roll motions are negligible. In this mode they are "platforming."
At a critical sea state specific to each design the vessels begin to change attitude in relation to the relative wave pattern. In this mode the vessels are "contouring." Contouring is a degradation of the superior swath motion characteristics with increasing pitch, heave and roll motion and is to be avoided when possible. The limiting critical sea state of these vessels is a direct function of the clearance between the underside of the upper bridge deck and the waterline. When the local wave height exceeds this clearance the vessel must begin contouring or the underside of the bridge deck will collide with potentially damaging wave tops.
Inherent in a swath design is a requirement for the bridging deck where the crew, passengers and equipment are located to be a significant height above the waterline. For a small swath vessel of 60 to 80 feet this height might be six feet or more. For a large swath of 300 feet this height might be 25 feet or more. This is problematic where people and equipment must be moved from the working deck to the surface of the water. Such movements may be required for immersing people or equipment in the water or moving people or equipment onto another vessel or shore-side structure such as a dock.
Current solutions for moving people and equipment from the working decks of swaths or semisubmerged ships to the waterline are limited. Of the seventeen swaths operational in the world today more than half have no fixed capability for moving people and equipment to the waterline. These ships must rely on hanging rope ladders for a minimal capability required of all ships for the recovery of crewmen who have fallen overboard.
Three swaths have "moonpools" installed which are structural cut-outs on the centerline of the vessel amidships. Through these cutouts relatively heavy equipment can be lowered either by crane or by platforms secured by wire hoists. The primary advantages of moon pools are the ample deck space adjacent to the moon pool and the negligible effect on a swath's heel or trim. Moonpools carry two great costs in terms of required supporting ships structure and loss of usable enclosed deck space. The latter is especially significant for smaller swaths with limited space.
At least five swaths operational today have the capability to lift and lower equipment over the side with cranes and A-frames. This is problematic for swaths because the small water plane area provides very little hydrostatic restoring moment especially in the transverse direction. This allows the vessel to obtain large angles of heel when relatively light weight equipment is cantilevered off to one side of the vessel.
None of the swaths operational today have satisfactory systems for moving divers or swimmers from the deck to the waterline frequently and efficiently. Commercial divers are lowered on platforms suspended by cranes or inside diving bells also suspended by cranes. No swaths operational today have any system for moving recreational divers and swimmers from the deck to the waterline.
Existing monohull technology is insufficient in providing solutions. Various swim platforms developed for smaller vessels such as those in U.S. Pat. No. 3,613,137 by Eccles (1971), U.S. Pat. No. 3,857,127 by Hedrickson (1974), U.S. Pat. No. 4,462,485 by Terry and Ritten (1984) and U.S. Pat. No. 4,548,155 by Hegg et al. (1985) utilize platforms attached to the transom or side at a fixed height. For a swath, the platform needs to be at the waterline only when the vessel is stopped and people or equipment need to be moved from the deck to the waterline. Before the vessel gets underway the platform must be removed or elevated so that waves passing under the bridge deck will not wash the platform away.
Extensible ladders have been proposed such as in U.S. Pat. No. 3,774,720 by Hovey (1973), U.S. Pat. No. 4,538,314 by Baranowski (1985), U.S. Pat. No. 4,733,752 by Sklar (1988), U.S. Pat. No. 4,724,925 by Ritten (1988), and U.S. Pat. No. 4,823,910 by Day (1989). For even the smallest swath the ladder required would have to be cantilevered from the hull six feet before becoming subject to wave forces. Such a configuration would require either a hinged ladder or excessive support structure.
Numerous accommodation ladder designs have been developed such as those disclosed in U.S. Pat. Nos. 3,841,439 and 3,856,110 by P. Nilsson (1974), U.S. Pat. No. 4,043,288 by Vulovic (1977), U.S. Pat. No. 4,115,887 by Ewards (1978) and U.S. Pat. No. 4,363,150 by E. Nilsson (1982). All of these designs used the expansive sides of a large ship for attachment and operation. For a swath the stern is the preferred location as the longitudinal hydrostatic stability of the swath configuration is inherently greater than the transverse hydrostatic stability resulting in less trim than heel for the same moment due to eccentric loading.
In U.S. Pat. No. 4,085,473 Franklin (1978) discloses a deck platform extension for boats utilizing a removable deck platform which attaches to the transom of a monohull at a fixed height with triangular support braces on the bottom side. The platform is not height adjustable nor is it designed to allow people or equipment to disembark onto a dock or second vessel or enter the water.
In U.S. Pat. No. 4,166,517 Henderson (1979) discloses a power elevator utilizing a cab guided along two tracks attached to the flat expansive side of the ship from the deck to the waterline and a winch and cable for raising and lowering. This idea would be difficult to implement on a swath since flat sides are usually nonexistent. The winch and cable means for moving the cab up and down could be improved on because of the inability of the cable to carry loads in compression. After the cab is lifted by a passing wave it will drop down to its original position and stop abruptly at the end of the cable run shock loading the cable and passengers. The cab can also be improved on by enlarging it allowing more people and a variety of equipment.